Evaporatively cooled fuel cell system and method for operating an evaporatively cooled fuel cell system

ABSTRACT

A fuel cell system ( 10 ) comprises a fuel cell ( 14 ) and an evaporative cooling system ( 16 ), which is in thermal contact with the fuel cell ( 14 ), in order that heat generated by the fuel cell ( 14 ) during operation of the fuel cell ( 14 ) is absorbed through evaporation of a cooling medium and is removed from the fuel cell ( 14 ). The fuel cell system ( 10 ) further comprises a device ( 22 ) for sensing the pressure in the evaporative cooling system ( 16 ). A control unit ( 24 ) is adapted to control the operating temperature of the fuel cell ( 14 ) in dependence on signals that are supplied to the control unit ( 24 ) from the device ( 22 ) for sensing the pressure in the evaporative cooling system ( 16 ), in such a way that the cooling medium of the evaporative cooling system ( 16 ) is transferred from the liquid to the gaseous state of matter by the heat generated by the fuel cell ( 14 ) during operation of the fuel cell ( 14 ).

The present invention relates to an evaporatively cooled fuel cellsystem and to a method for operating an evaporatively cooled fuel cellsystem.

Fuel cell systems enable electrical power to be generated with lowemissions and high efficiency. For this reason, efforts are being madeat present to apply fuel cell systems in various mobile applications,such as, for example, in automobile engineering, in shipping or inaviation, for the purpose of generating electrical energy. For example,in an aircraft, it is conceivable for the generators currently used forthe on-board electrical power supply, which are driven by the mainengines or the auxiliary turbine, to be replaced by a fuel cell system.Moreover, a fuel cell system could also be used for the emergencyelectrical power supply of the aircraft, and replace the ram air turbine(RAT) used hitherto.

Fuel cells usually comprise a cathode region, and an anode region, whichis separated from the cathode region by an electrolyte. When the fuelcell is in operation, a fuel, for example hydrogen, is supplied to theanode side of the fuel cell, and an oxygen-containing oxidant, forexample air, is supplied to the cathode side of the fuel cell. In apolymer electrolyte membrane (PEM) fuel cell, the hydrogen moleculesreact, at an anode catalyst present in the anode region, for exampleaccording to equation (1)

H₂→2·H⁺+2 ·e ⁻  (1)

and, by forming positively charged hydrogen ions, thereby deliverelectrons to the electrode.

The H⁺ ions formed in the anode region then diffuse through theelectrolyte to the cathode, where, at a cathode catalyst present in thecathode region, they react with the oxygen supplied to the cathode andwith the electrons routed to the cathode via an external electricalcircuit, according to equation (2)

0.5·O₂+2·H⁺+2·e ⁻→H₂O  (2)

to form water.

In addition to generating electrical energy, a fuel cell, when inoperation, generates thermal energy, which must be removed from the fuelcell with the aid of a cooling system, in order to prevent overheatingof the fuel cell. In the case of mobile applications, in which usuallyonly a portion of the thermal energy generated by the fuel cell duringoperation can be supplied to in-system or external heat sinks forfurther use, frequently at least a portion of the heat of reactiongenerated by the fuel cells must be emitted to the environment. A fuelcell used in an aircraft, for example for the on-board electrical powersupply, must be so designed that it is capable of fulfilling a largerequirement for electrical energy. However, a fuel cell that has a highcapacity in respect of generating electrical energy also generates alarge quantity of thermal energy, and therefore has a high coolingrequirement.

In principle, a fuel cell used on board an aircraft can be cooled invarious ways. For example, liquid cooling is possible, wherein a liquidis used as a cooling medium, in order to absorb the heat of reactiongenerated by the fuel cell. The cooling capacity of a liquid coolingsystem is calculated roughly according to equation (3)

{dot over (Q)} _(F) ={dot over (m)} _(F) ·c _(pF) ·ΔT _(F)  (3)

wherein {dot over (Q)}_(F) is the heat absorption capacity of thecooling liquid, {dot over (m)}_(F) is the mass flow rate, c_(pF) is thethermal capacity of the cooling liquid, and ΔT_(F) is the temperaturedifference between the cooling-liquid outlet temperature and thecooling-liquid inlet temperature.

As is directly evident from equation (3), effective liquid cooling, inwhich the cooling liquid is routed in a circuit, requires that coolingliquid heated through absorption of heat from the fuel cell be cooleddown again by ΔT_(F), before it can again effectively absorb waste heatfrom the fuel cell. For the purpose of cooling down the cooling liquidby ΔT_(F), the cooling liquid can be supplied, for example, to a heatexchanger, in which the thermal energy stored in the cooling liquid istransferred to a further cooling medium, for example ambient air. As analternative to liquid cooling of the fuel cell with subsequentambient-air recooling of the cooling liquid, direct ambient-air coolingof the fuel cell is also conceivable.

Irrespective of whether an ambient-air cooling system is used for directambient-air cooling of a fuel cell or serves only to recool the coolingliquid of a liquid cooling system, the cooling capacity of theambient-air cooling system is calculated roughly according to equation(4)

{dot over (Q)} _(L) ={dot over (m)} _(L) ·c _(pL) ·ΔT _(L)  (4)

wherein {dot over (Q)}_(L) is the heat absorption capacity of thecooling air, {dot over (m)}_(L) is the mass flow rate of the coolingair, c_(pL) is the thermal capacity of the cooling air, and ΔT_(L) isthe temperature difference between the cooling-air outlet temperatureand the ambient-air temperature.

Equation (4) makes clear that, the smaller the temperature differenceΔT_(L) between the cooling-air outlet temperature and the ambient-airtemperature, the lesser is the cooling capacity of ambient-air cooling.In the case of the cooling of a low-temperature PEM fuel cell, whoseoperating temperature is usually between approximately 60 and 110° C.,with an output optimum between approximately 60 and 90° C., theretherefore exists the problem that the temperature difference ΔT_(L)between the cooling-air outlet temperature, corresponding maximally tothe operating temperature of the fuel cell, and the ambient-airtemperature is relatively small, and the cooling capacity of the coolingsystem is therefore correspondingly small. Consequently, large heattransfer surfaces are required for adequately removing heat from a PEMfuel cell to the environment. Liquid and/or air cooling systems for PEMfuel cells are therefore necessarily of large volume and of relativelygreat weight, which is very disadvantageous for use in mobileapplications, and particularly in aviation. Moreover, cold air requiredfor cooling a PEM fuel cell used on board an aircraft has to be suckedin from the aircraft environment and, following the absorption of theheat of reaction generated by the fuel cell, removed back into theaircraft environment. However, both the suction intake of the air andits removal into the aircraft environment causes increased airresistance, which is disadvantageous for efficient flight operation ofthe aircraft.

In contrast to the cooling systems described above, the cooling capacityof which depends substantially on the difference between the temperatureof the cooling medium and the ambient-air temperature, the coolingcapacity of an evaporative cooling system according to equation (5)

{dot over (Q)} _(V) ={dot over (m)} _(V) ·c _(pL) ·Δh _(V)  (5)

is determined by the evaporation enthalpy Δh_(V) of a cooling mediumused in the evaporative cooling system, {dot over (Q)}_(V) being theheat absorption capacity of the cooling medium to be transferred fromthe liquid to the gaseous state of matter, and {dot over (m)}_(V) beingthe mass flow rate of the cooling medium to be transferred from theliquid to the gaseous state of matter.

Following evaporation, a cooling medium routed in a circuit in anevaporative cooling system does have to be transferred back to itsliquid state of matter through condensation. It is not necessary,however, for the cooling medium to be cooled to a temperature that isbelow the operating temperature of a fuel cell cooled by the evaporativecooling system. Moreover, compared with the liquid-cooling andair-cooling systems described above, an evaporative cooling system hasthe advantage that the change of state of conventional cooling mediasuch as, for example, water, requires very much more energy than thecooling medium is capable to absorb in the liquid state. An evaporativecooling system can therefore be operated with a significantly lessercooling-medium mass flow rate than an air cooling system having acomparable cooling capacity.

An evaporative cooling system for cooling a fuel cell is known, forexample, from DE 199 35 719 A1. In the cooling system described in DE199 35 719 A1, a cooling medium, for example water, is routed throughcooling pipes, which are arranged in an anode gas-supply chamber and ananode exhaust-gas chamber of a fuel-cell stack. The cooling mediumevaporates as it flows through the cooling pipes, and thereby absorbs upto 90% of the quantity of heat emitted from the fuel cell throughthermal radiation.

The present invention is directed to the object of providing anevaporatively cooled fuel cell system that is suitable, in particular,for use in an aircraft. Further, the invention is directed to the objectof providing a method for operating such an evaporatively cooled fuelcell system.

This object is achieved by a fuel cell system having the featuresspecified in claim 1, and by a method, having the features specified inclaim 11, for operating a fuel cell system.

A fuel cell system according to the invention comprises a fuel cell,preferably a fuel cell of the MW output class, the term “fuel cell” heredenoting not only a single cell, but also a fuel-cell stack comprising amultiplicity of fuel cells. The fuel cell is preferably a PEM fuel cell,the anode region of which is connected to a hydrogen source, and thecathode side of which is supplied with an oxygen-containing oxidationmeans, preferably air. The fuel to be supplied to the anode region ofthe fuel cell, preferably hydrogen, can be stored in a fuel tankintegrated into the fuel cell system according to the invention.Alternatively or in addition thereto, the fuel cell system according tothe invention can comprise a fuel generating installation for generatingthe fuel to be supplied to the anode region of the fuel cell. Thecathode side of the fuel cell can be connected to a pressure side of acompressor. The compressor can be a compressor having a combined air andvapour inlet.

The fuel cell can be a low-temperature PEM fuel cell or ahigh-temperature PEM fuel cell, a PEM fuel cell containing a compositeelectrolyte having inorganic material, a polybenzimidazole PEM fuel cellor a polyperfluorsulfonic acid PEM fuel cell. When in operation, thefuel cell, in addition to producing electrical energy, produces thermalenergy, which must be removed from the fuel cell in order to prevent thefuel cell from overheating.

The fuel cell system according to the invention therefore furthercomprises an evaporative cooling system, which is in thermal contactwith the fuel cell, in order that heat generated by the fuel cell duringoperation of the fuel cell is absorbed through evaporation of a coolingmedium and is removed from the fuel cell. As explained above, anevaporative cooling system is distinguished by a high heat absorptioncapacity, and consequently an excellent cooling capacity, owing to thehigh evaporation enthalpy required for bringing the cooling medium fromthe liquid to the gaseous state of matter. The evaporative coolingsystem of the fuel cell system according to the invention can thereforebe operated with a significantly lesser cooling-medium mass flow rate,compared with an air or liquid cooling system having a similar heatabsorption capacity, and, moreover, is of a compact design and lowerweight. Further, the evaporative cooling system has low energy losses,does not cause any additional air resistance when the fuel cell systemaccording to the invention is used on board an aircraft and also duringoperational load peaks is capable of reliably and autonomously supplyingthe fuel cell with cooling capacity. Finally, a high system dynamic canbe realized in the evaporative cooling system of the fuel cell systemaccording to the invention, since the expansion of the cooling mediumupon evaporation enables the cooling medium to be rapidly removed fromthe region of the evaporative cooling system that is in thermal contactwith the fuel cell.

The evaporative cooling system of the fuel cell system according to theinvention can be realized as a system designed separate from the fuelcell. Preferably, however, the evaporative cooling system is at leastpartially integrated into components of the fuel cell, and comprises,for example, cooling channels formed in bipolar plates, separatorplates, cover plates and/or lateral delimiting plates of the fuel cellor fuel-cell stack. Alternatively or in addition thereto, coolingchannels of the evaporative cooling system can also extend in an anodegas supply chamber, a cathode gas supply chamber, an anode exhaust-gaschamber and/or a cathode exhaust-gas chamber of the fuel cell. A coolingmedium flows through the cooling channels of the evaporative coolingsystem, which cooling medium is transferred from the liquid to thegaseous state of matter through absorption of the heat generated by thefuel cell during operation. Water, for example, can be used as a coolingmedium, which is evaporated by nucleate boiling as it flows through thecooling channels of the evaporative cooling system.

The fuel cell system according to the invention further comprises adevice for sensing the pressure in the evaporative cooling system, i.e.in the parts of the evaporative cooling system in which the coolingmedium is transferred from the liquid to the gaseous state of matter. Ifthe evaporative cooling system, i.e. the parts of the evaporativecooling system in which the cooling medium is transferred from theliquid to the gaseous state of matter, is/are connected to theenvironment of the evaporative cooling system, the pressure sensingdevice can be adapted and so arranged that it senses the ambientpressure in the environment of the evaporative cooling system and or thefuel cell. A pressure sensor, for example, can be used as the pressuresensing device. The pressure sensing device supplies signals, which arecharacteristic of the pressure in the evaporative cooling system, to acontrol unit, which is realized, for example, as an electronic controlunit.

The control unit of the fuel cell system according to the invention isadapted to control the operating temperature of the fuel cell independence on the signals that are supplied to the control unit from thedevice for sensing the pressure in the evaporative cooling system, insuch a way that the cooling medium of the evaporative cooling system istransferred from the liquid to the gaseous state of matter by the heatgenerated by the fuel cell during operation of the fuel cell. In otherwords, the control unit is adapted so to control the operatingtemperature of the fuel cell in dependence on the pressure in theevaporative cooling system, i.e. in the parts of the evaporative coolingsystem in which the cooling medium is evaporated, such that it is alwaysensured that the heat generated by the fuel cell during operation of thefuel cell is sufficient to transfer the cooling medium of theevaporative cooling system from the liquid to the gaseous state ofmatter. As a result, proper functioning of the evaporative coolingsystem is always ensured. In the fuel cell system according to theinvention, the temperature of the cooling medium after absorption of theheat generated by the fuel cell is just under the operating temperatureof the fuel cell, such that a high heat transfer, and thus aparticularly good cooling capacity of the evaporative cooling system, isrealized. Moreover, the substantially isothermal change of state ofmatter of the cooling medium renders possible stable operation of theevaporative cooling system.

The evaporation temperature of usual cooling media such as, for example,water, decreases as pressure decreases. For example, the evaporationtemperature of water is 100° C. at a pressure corresponding to theatmospheric pressure at sea-level (1.0132 bar). By contrast, at apressure of 0.1992 bar, as exists at an altitude of 12192 m (40000feet), i.e. the cruising altitude of a commercial aircraft, theevaporation temperature of the water is only 60° C. Consequently, if theevaporative cooling system of a fuel cell system according to theinvention is operated with water as the cooling medium, in the case of apressure in the evaporative cooling system corresponding to theatmospheric pressure at sea-level, the operating temperature of the fuelcell must be selected to be so high that the heat generated by the fuelcell during operation is sufficient to heat the cooling medium of theevaporative cooling system to over 100° C. and thereby to provide forproper functioning of the evaporative cooling system. If, on the otherhand, the pressure in the evaporative cooling system is only 0.1992 bar,the fuel cell can be operated with a lesser operating temperature, sincethe heat generated by the fuel cell during operation only has to besufficient to heat the cooling medium to 60° C.

A low-temperature PEM fuel cell attains its output optimum when it isoperated at an operating temperature of between approximately 60 and 90°C. If, in the fuel cell system according to the invention, it isdetermined, with the aid of the pressure sensing device, that there is asufficiently low pressure in the evaporative cooling system, for exampledue to an ambient pressure which is lower than the atmospheric pressureat sea-level, the control unit can lower the operating temperature ofthe fuel cell to such an extent that an optimum output of the fuel cellis achieved, but the heat generated by the fuel cell during operation isstill sufficient to transfer the cooling medium of the evaporativecooling system from the liquid to the gaseous state of matter andthereby to ensure proper functioning of the evaporative cooling system.

When in flight, an aircraft is predominantly in an environment in whichthe ambient pressure is below the atmospheric pressure at sea-level.This fact can be used, in a particularly advantageous manner, in a fuelcell system according to the invention used on board an aircraft, tokeep the operating temperature of the fuel cell below 100° C., butinsofar as possible in the optimum operating temperature range ofbetween approximately 60 and 90° C., for the majority of the operatingperiod of the fuel cell. This requires only that the evaporative coolingsystem, i.e. the parts of the evaporative cooling system in which thecooling medium is evaporated, be arranged in the non-pressurized regionof the aircraft and be connected to the ambient atmosphere, such thatthe lower ambient pressure in the environment of the aircraft is presentin these parts of the evaporative cooling system.

For example, the control unit can keep the operating temperature of thefuel cell of a fuel cell system according to the invention used on boardan aircraft at a constant temperature, insofar as possible in theoptimum operating temperature range of the fuel cell, for as long as theaircraft is at a constant cruising altitude. On the other hand, when theaircraft is in ascent, the control unit can lower the operatingtemperature of the fuel cell in dependence on the decreasing ambientpressure, whereas, when the aircraft is in descent, it can increase theoperating temperature of the fuel cell in dependence on the increasingambient pressure.

If water is used as a cooling medium in the evaporative cooling systemof the fuel cell system according to the invention, the control unit cankeep the operating temperature of the fuel cell at, for example,approximately 60° C., when the aircraft is at a constant cruisingaltitude of approximately 12192 m (40000 feet). When the aircraft is inascent, from a starting location at approximately sea-level, until thecruising altitude is attained, the control unit can reduce the operatingtemperature of the fuel cell continuously during the ascent, fromapproximately 100° C. to approximately 60° C., for example in dependenceon the decreasing ambient pressure. On the other hand, when the aircraftis in descent, from the cruising altitude to a landing location atapproximately sea-level, the control unit can increase the operatingtemperature of the fuel cell continuously from approximately 60° C. toapproximately 100° C., in dependence on the increasing ambient pressure.

In a preferred embodiment of the fuel cell system according to theinvention, the control unit is adapted to control the operatingtemperature of the fuel cell in dependence on the signals that aresupplied to the control unit from the device for sensing the pressure inthe evaporative cooling system, in such a way that the evaporation ofthe cooling medium of the evaporative cooling system by the heatgenerated by the fuel cell during operation of the fuel cell is effectedin the wet-steam region of the cooling medium. “Wet steam” here isunderstood to be a system in which boiling liquid and saturated steamare in equilibrium.

If the evaporation of the cooling medium used in the evaporative coolingsystem of the fuel cell system according to the invention is effected inthe wet-steam region of the cooling medium, during the evaporation ofthe cooling medium, boiling cooling medium, in the liquid state ofmatter, is in equilibrium with saturated steam of the cooling medium.This is the case whenever the cooling medium, during evaporation, isheated to its pressure-dependent evaporation temperature. The controldevice thus preferably controls the operating temperature of the fuelcell such that the cooling medium of the evaporative cooling system isheated to a temperature corresponding to the pressure-dependentevaporation temperature of the cooling medium. For example, the fuelcell can be operated at an operating temperature that is 0 to 5° C.,preferably 1 to 3° C., above the pressure-dependent evaporationtemperature of the cooling medium of the evaporative cooling system.

The fuel cell system according to the invention preferably furthercomprises a fuel-cell operating-pressure generating system, which isadapted to generate a desired pressure in the fuel cell, i.e. in thecomponents of the fuel cell in which there are no integrated coolingchannels of the evaporative cooling system. The fuel-celloperating-pressure generating system of the fuel cell system accordingto the invention can be integrated, for example, into a media supplysystem of the fuel cell system, and comprise a compressor for supplyingan oxidant into the cathode region of the fuel cell and/or comprise acorresponding delivery device for supplying fuel into the anode regionof the fuel cell. The fuel-cell operating-pressure generating systemserves to bring the fuel-cell operating pressure to a desired level, orto keep it at a desired level, irrespective of the ambient pressure inthe environment of the fuel cell and irrespective of the pressure in theevaporative cooling system.

The fuel-cell operating-pressure generating system can be adapted togenerate in the fuel cell a pressure that is lower or higher than theambient pressure in the environment of the fuel cell and/or than thepressure in the evaporative cooling system. Further, the fuel cellsystem according to the invention can comprise a control unit realized,for example, as an electronic control unit, for controlling thefuel-cell operating-pressure generating system. The control unit forcontrolling the fuel-cell operating-pressure generating system can be aseparate control unit. As an alternative thereto, however, the controlunit for controlling the fuel-cell operating-pressure generating systemcan also be integrated into the control unit for controlling theoperating temperature of the fuel cell.

As explained above, in the fuel cell system according to the inventionthe operating temperature of the fuel cell is always controlled, independence on the pressure in the evaporative cooling system, such thatthe cooling medium of the evaporative cooling system is brought from theliquid to the gaseous state by the heat generated by the fuel cellduring operation of the fuel cell. In the case of correspondingly highoperating temperatures, however, the problem can arise that substancesand/or substance mixtures such as, for example, water, that are usuallypresent in liquid form in the fuel cell, i.e., for example, in the anoderegion, the cathode region, in the region of a membrane separating theanode region from the cathode region, in the anode gas lines or thecathode gas lines, evaporate.

In order to prevent an unwanted evaporation of substances and/orsubstance mixtures usually present in liquid form in the fuel cell fromduring operation of the fuel cell, the control unit for controlling thefuel-cell operating-pressure generating system can be adapted to controlthe fuel-cell operating-pressure generating system such that there isgenerated in the fuel cell a pressure at which unwanted evaporation ofsubstances and/or substance mixtures usually present in liquid form inthe fuel cell is prevented.

The control unit for controlling the fuel-cell operating-pressuregenerating system can be adapted to control the fuel-cell operatingpressure in dependence on the operating temperature of the fuel cell.The operating temperature of the fuel cell that is used by the controlunit as a control variable for controlling the fuel-celloperating-pressure generating system can be a set operating temperatureof the fuel cell provided by the control unit for controlling theoperating temperature of the fuel cell, or it can be an operatingtemperature of the fuel cell that is measured, for example, by means ofa temperature sensor. Alternatively or in addition thereto, however, thecontrol unit for controlling the fuel-cell operating-pressure generatingsystem can also be adapted to control the fuel-cell operating pressurein dependence on signals supplied to it from the device for sensing thepressure in the evaporative cooling system. For example, the controlunit for controlling the fuel-cell operating-pressure generating systemcan calculate a set operating temperature of the fuel cell on the basisof the signals of the pressure sensing device that are characteristic ofthe pressure in the evaporative cooling system, and use it as a controlvariable for the determination of an appropriate set operating pressurein the fuel cell.

If water is used as an environmentally benign cooling medium in theevaporative cooling system of the fuel cell system according to theinvention, the fuel cell, realized, for example, as a PEM fuel cell, ispreferably operated, in the case of a pressure in the evaporativecooling system corresponding approximately to the atmospheric pressureat sea-level, at an operating temperature of approximately 100 to 105°C. The control unit for controlling the fuel-cell operating-pressuregenerating system then preferably so controls the fuel-celloperating-pressure generating system that a pressure above theatmospheric pressure at sea-level, for example of 2 bar, is generated inthe fuel cell. At a pressure of 2 bar, the evaporation temperature ofwater is 120.23° C., such that evaporation of water present in the fuelcell, i.e., for example, in the anode region, in the cathode region, inthe region of a membrane separating the anode region from the cathoderegion, in the anode gas lines or the cathode gas lines, is reliablyprevented at the operating temperature of the fuel cell.

As explained above, large quantities of heat can be removed from thefuel cell through the evaporation process taking place in theevaporative cooling system of the fuel cell system according to theinvention. The heat removed from the fuel cell by the evaporativecooling system must then either be emitted to the environment orsupplied to a further use. A heat transfer process can be described byequation (6)

{dot over (Q)}=k·A·Δt _(m log)  (6)

wherein {dot over (Q)} is the transferred heat, k is the heat transfercoefficient, A is the heat transfer surface and Δt_(m log) is thetemperature gradient. The heat transfer coefficient k is calculatedaccording to equation (7)

1/k=1/α_(out) +s/λ+1/α_(in)  (7)

wherein s is the wall thickness, λ is the thermal conductivitycoefficient and α is the heat transfer coefficient.

The heat transfer coefficient α is regarded as the main variableinfluencing the heat output {dot over (Q)} to be transferred in thecourse of a heat transfer process. A large heat transfer coefficient αis achieved in the case of a condensation process. The evaporativecooling system of the fuel cell system according to the inventiontherefore preferably comprises a condenser, for condensing the coolingmedium evaporated during operation of the fuel cell for the purpose ofcooling the fuel cell. In the case of such a design of the evaporativecooling system, absorption of the heat of reaction of the fuel cell iseffected through evaporation, whereas the emission of the heat ofreaction of the fuel cell is realized through a condensation process.Since a condensation process, in a manner similar to an evaporationprocess, consumes very much more energy than, for example, a coolingmedium in liquid form is able to absorb, an evaporative cooling systemprovided with a condenser operates particularly efficiently.

A further advantage of an evaporative cooling system provided with acondenser consists in that it can be operated as a circuit system, inwhich cooling medium that is condensed in the condenser is returned, ina liquid state of matter, back to the fuel cell, where it can beevaporated again for the purpose of cooling the fuel cell. Preferably,however, in an evaporative cooling system provided with a condenser, thefuel-cell cooling function is decoupled from the cooling-medium recoveryfunction, such that adequate cooling of the fuel cell is ensured even inthe event of failure of the condenser. In order to ensure that theevaporative cooling system is adequately supplied with cooling medium,the fuel cell system according to the invention can comprise anapparatus for supplying water produced during operation of the fuel cellinto the evaporative cooling system.

The heat removed from the fuel cell by means of the evaporative coolingsystem can be emitted to the environment. This is particularlyappropriate when the resultant heat is at a comparatively lowtemperature level of, for example, 60° C.

A condenser that enables waste heat generated by the fuel cell of thefuel cell system according to the invention to be efficiently removed tothe environment can be realized, for example, in the form of anouter-skin cooler. The outer-skin cooler can be constituted, forexample, by a wall, of which the inside, facing towards the fuel cellsystem, receives applied vaporous cooling medium and operates as avapour condenser. On the other hand, an outside of the wall constitutingthe outer-skin cooler, which outside faces towards the environment,operates as an ambient-air heating means. A condenser realized as anouter-skin cooler is appropriate for use, in particular, in a fuel cellsystem according to the invention used on board an aircraft. Theouter-skin cooler can then be constituted, for example, by a portion ofthe aircraft is outer skin, which receives, on its inside, appliedcooling medium that is evaporated during operation of the fuel cell forthe purpose of cooling the fuel cell.

If a portion of the aircraft's outer skin is used as an outer-skincooler, large heat transfer surfaces can be created easily, and withoutadditional components. This results in a considerable advantage inrespect of weight. Moreover, an outer-skin cooler, constituted, forexample, by a portion of the aircraft's outer skin, is distinguished bya high cooling capacity, and enables further advantages to be achievedin respect of weight, owing to the absence of pipelines. Moreover, theremoval of heat by means of an outer-skin cooler produces little noise,and it does not require any large air-mass movements, which, in the caseof the fuel cell system according to the invention being used in anaircraft, could result in unwanted, additional air resistance. Finally,disturbance of the loft is avoided.

As an alternative or in addition to the heat of reaction, produced bythe fuel cell of the fuel cell system according to the invention duringoperation, being removed to the environment, the heat of reactiongenerated by the fuel cell can also be recovered and utilized. For thispurpose, the fuel cell system according to the invention can comprise atleast one device for utilizing heat stored in the cooling medium. Theutilization of the heat stored in the cooling medium can be effecteddirectly or indirectly. For example, the cooling medium, in the gaseousstate of matter, can be supplied directly to the device for utilizingthe heat stored in the cooling medium. However, cooling medium drawnfrom the evaporative cooling system in this process must be conveyedback into the evaporative cooling system in order to ensure propercooling of the fuel cell.

Alternatively or additionally, it is also conceivable for only the heatstored in the cooling medium to be transferred to the device forutilization of this heat. For this purpose, the cooling medium, in thegaseous state of matter, can be routed, for example, through a heatexchanger, which is in thermal contact with the device for utilizing theheat stored in the cooling medium. Further, a condenser provided in theevaporative cooling system can be so realized and/or arranged that theheat released upon the condensation of the cooling medium in thecondenser is transferred to the device for utilizing the heat stored inthe cooling medium.

The device for utilizing the heat stored in the cooling medium can be aheating device, preferably realized as steam heating means, whichutilizes for heating purposes the heat stored in the cooling medium. Asan alternative thereto, however, the device for utilizing the heatstored in the cooling medium can also be a water desalinationinstallation, for obtaining drinking water from sea water. In a fuelcell system according to the invention provided for use in an aircraft,the device for utilizing the heat stored in the cooling medium ispreferably a de-icing installation of the aircraft.

Finally, it is conceivable for the water used as a cooling medium in theevaporative cooling system to be utilized in an installation forsupplying water and/or water vapour into an exhaust-gas stream of anaircraft. It is thereby possible to reduce the pollutant emission of anaircraft engine.

A preferred embodiment of the fuel cell system according to theinvention preferably further comprises an apparatus for removing thecooling medium to the environment. Preferably, such an apparatus forremoving the cooling medium to the environment is adapted to remove tothe environment the cooling medium in the gaseous state of matter, ifthe heat stored in the cooling medium cannot be removed to theenvironment, or otherwise utilized, to a sufficient extent. Theapparatus for removing the cooling medium to the environment can berealized, for example, in the form of an outlet valve, and ensuresreliable operation of the fuel cell system according to the inventioneven if the removal of the heat stored in the cooling medium is renderedmore difficult, for example as a result of high ambient temperature orconditions of calm.

In a method, according to the invention, for operating a fuel cellsystem comprising a fuel cell and an evaporative cooling system, whichis in thermal contact with the fuel cell, in order that heat generatedby the fuel cell during operation of the fuel cell is absorbed throughevaporation of a cooling medium and is removed from the fuel cell, thepressure in the evaporative cooling system is sensed by means of anappropriate pressure sensing device. The operating temperature of thefuel cell is controlled by means of a control unit, in dependence onsignals that are supplied to the control unit from the pressure sensingdevice. Control of the operating temperature of the fuel cell iseffected in such a manner that the cooling medium of the evaporativecooling system is transferred from the liquid to the gaseous state ofmatter by the heat generated by the fuel cell during operation of thefuel cell.

Preferably, the operating temperature of the fuel cell is controlled, independence on the signals supplied to the control unit from the pressuresensing device, in such a way that the evaporation of the cooling mediumof the evaporative cooling system by the heat generated by the fuel cellduring operation of the fuel cell is effected in the wet-steam region ofthe cooling medium.

In a preferred embodiment of the method, according to the invention, foroperating a fuel cell system, a desired pressure is generated in thefuel cell by means of a fuel-cell operating-pressure generating system.For example, a desired pressure is generated, by means of the fuel-celloperating-pressure generating system, in the anode region, in thecathode region, in the region of a membrane separating the anode regionfrom the cathode region, in the anode gas lines and the cathode gaslines of the fuel cell.

The fuel-cell operating-pressure generating system, by means of acontrol unit for controlling the fuel-cell operating-pressure generatingsystem, can be controlled such that there is generated in the fuel cella pressure at which unwanted evaporation of substances and/or substancesmixtures usually present in liquid form in the fuel cell is prevented.

Preferably, the control unit for controlling the fuel-celloperating-pressure generating system controls the pressure in the fuelcell in dependence on the operating temperature of the fuel cell and/orin dependence on the signals of the pressure sensing device for sensingthe pressure in the evaporative cooling system.

The cooling medium evaporated during operation of the fuel cell for thepurpose of cooling the fuel cell can be condensed in a condenser.Preferably, the cooling medium is condensed by a condenser realized inthe form of an outer-skin cooler.

Alternatively or in addition thereto, heat stored in the cooling mediumcan also be supplied to at least one device for the utilization of thisheat. For example, the heat stored in the cooling medium can be suppliedto a device for utilizing this heat that is realized in the form of asteam heating means, a water desalination installation or a de-icinginstallation of an aircraft.

Further, water used as a cooling medium in the evaporative coolingsystem can also be supplied, in the form of liquid or vapour, into anexhaust-gas stream of an aircraft.

Preferably, the cooling medium is removed to the environment ifrequired, i.e. when proper removal or utilization of the heat stored inthe cooling medium is not possible.

The fuel cell system according to the invention is particularly suitablefor use as a fuel-cell based energy supply unit in an aircraft,particularly a plane. For example, the fuel cell system can be used asan alternative to the auxiliary power unit (APU) or the ram air turbine(RAT), or as an energy supply system for a wing anti-ice system (WAIS).In order to cover the electrical power requirement in an aircraft, theremay be a requirement for a fuel cell system capable of generating 1 MWof electrical power. The evaporative cooling system of the fuel cellsystem according to the invention is capable, despite its low weight andits compact design, of providing sufficient cooling capacity for coolinga high-power fuel cell. Moreover, autonomous operation of the coolingsystem is possible, independently of other aircraft systems, such as,for example, an aircraft air-conditioning installation. The fuel cellsystem according to the invention can therefore be used on board anaircraft, for example as an autonomous emergency electrical power supplyunit.

In an aircraft equipped with the fuel cell system according to theinvention, the fuel cell system, or at least the parts of theevaporative cooling system in which the cooling medium is evaporated,is/are preferably arranged in a non-pressurized region of the aircraft.Such regions in the aircraft are located, for example, in the mainstructure/fuselage covering (belly fairing) and in the fuselage aftsection.

Three preferred exemplary embodiments of a fuel cell system according tothe invention are now explained more fully with reference to theappended schematic figures, of which

FIG. 1 shows a schematic representation of a first embodiment of a fuelcell system,

FIG. 2 shows a schematic representation of a second embodiment of a fuelcell system, and

FIG. 3 shows a schematic representation of a third embodiment of a fuelcell system.

FIG. 1 shows a fuel cell system 10, which is arranged in the bellyfairing 12, i.e. in a non-pressurized region of an aircraft. The fuelcell system 10 comprises a fuel cell 14 realized in the form of afuel-cell stack. The fuel cell 14 is realized as a low-temperature PEMfuel cell having an operating temperature range of between 60 and 110°C. The optimum operating temperature range of the fuel cell 14 isbetween 60 and 90° C.

An evaporative cooling system 16 provided for cooling the fuel cell 14comprises cooling channels 17, which are realized in bipolar plates ofthe fuel cell 14 that are not represented in greater detail in FIG. 1.Water, serving as a cooling medium, flows through the cooling channels17 during operation of the fuel cell 14. The water, in a liquid state ofmatter, is supplied to the cooling channels 17 from a cooling-mediumtank 20, by means of a pump 18.

Further arranged in the interior of the belly fairing 12 is a pressuresensor 22. The pressure sensor 22 measures the pressure in the interiorof the belly fairing 12, which pressure corresponds to the pressure inthe evaporative cooling system 16, i.e. the pressure in the coolingchannels 17. As already mentioned, the belly fairing 12 belongs to thenon-pressurized regions of the aircraft, such that the pressure p₁present in the interior of the belly fairing 12 and measured by thepressure sensor 22 corresponds substantially to the ambient pressurep_(A) in the environment of the aircraft. When the aircraft is inflight, and particularly when the aircraft is at its cruising altitude,this pressure is significantly less than the atmospheric pressure atsea-level.

Signals emitted by the pressure sensor 22, which are characteristic ofthe ambient pressure in the interior of the belly fairing 12, aresupplied to an electronic control unit 24. On the basis of the signalsof the pressure sensor 22, the electronic control unit 24 controls theoperating temperature of the fuel cell 14 in such a manner that thewater is transferred from the liquid to the gaseous state of matter asit flows through the cooling channels 17 realized in the bipolar platesof the fuel cell 14. The evaporation of the water flowing through thecooling channels 17 causes the heat of reaction generated by the fuelcell 14 during operation to be absorbed by the water serving as acooling medium.

In particular, the electronic control unit 24 controls the operatingtemperature of the fuel cell 14, in dependence on the signals suppliedto the control unit 24 from the pressure sensor 22, in such a mannerthat the evaporation of the water flowing through the cooling channels17 of the evaporative cooling system 16 by the heat of reactiongenerated by the fuel cell 14 during operation is effected in thewet-steam region. In order to achieve this, the control unit 24 alwayscontrols the operating temperature of the fuel cell 14 such that theoperating temperature of the fuel cell 14 is about 0 to 5° C. above thepressure-dependent evaporation temperature of the cooling medium water.

The operating temperature of the fuel cell 14 is thus controlled by thecontrol unit 24, in dependence on the flight altitude of the aircraftand the resultant pressure in the belly fairing 12, such that it alwaysfollows the pressure-dependent evaporation curve of the cooling mediumwater. Table 1 gives corresponding values for the boiling or evaporationtemperature, and the boiling or evaporation pressure, of the coolingmedium water, in dependence on the flight altitude of the aircraft.

TABLE 1 Boiling temperature and boiling pressure of the cooling mediumwater in dependence on the flight altitude of the aircraft (northernhemisphere, 45° N, July) Boiling Boiling Flight temperature, pressure,altitude, [° C.] [bar] [m] 59 0.1901 12497 (41000 feet)  60 0.1992 12192(40000 feet)  70 0.3116 9144 (30000 feet) 80 0.4736 6096 (20000 feet) 900.7011 3048 (10000 feet) 100 1.0132 0 120.23 2 —

It is evident from Table 1 that the fuel cell 14 can be operated at anoperating temperature of approximately 60° C. when the aircraft is atcruising altitude. When the aircraft is in ascent and descent, theoperating temperature of the fuel cell 14 is adapted continuously to thevarying ambient pressure by the control unit 24. In other words, theoperating temperature of the fuel cell 14 is made to track the topressure-dependent evaporation curve of the cooling medium water in sucha way that the evaporation of the cooling medium water in the coolingchannels 17 of the evaporative cooling system 16, both during ascent anddescent of the aircraft, is effected in the wet-steam region. As aresult, proper evaporation of the cooling medium flowing through thecooling channels 17 of the evaporative cooling system 16 is alwaysensured. At the same time, operation of the fuel cell 14 atunnecessarily high operating temperatures is avoided.

Table 1 shows that the fuel cell 14 can be operated in the optimumoperating temperature range of from 60 to 90° C. over a large portion ofits operating time. Higher operating temperatures of the fuel cell 14are only necessary when the aircraft is flying at an altitude of under3048 m (10000 feet) or is on the ground.

In order to prevent unwanted evaporation of water usually present inliquid form in the fuel cell 14, i.e., for example, in an anode region,a cathode region, in the region of a membrane separating the anoderegion from the cathode region, in anode gas lines or cathode gas lines,the control unit 24 controls a fuel-cell operating-pressure generatingsystem 25, which is integrated into a media supply system of the fuelcell 14, in such a manner that there is generated in the fuel cell 14 anoperating pressure at which unwanted evaporation of the liquid waterpresent in the fuel cell is prevented. The low-temperature PEM fuel cell14 shown in FIG. 1 is operated at an operating pressure of 2 bar. At apressure of 2 bar, the evaporation temperature of water is 120.23° C.,such that evaporation of the liquid water present in the fuel cell 14 isreliably prevented in the entire operating temperature range of the fuelcell 14.

The fuel cell system 10 shown in FIG. 1 further comprises a condenser 26realized in the form of an outer-skin cooler. The condenser 26 serves tobring back to the liquid state of matter the water evaporated duringoperation of the fuel cell 14 for the purpose of cooling the fuel cell14. The condenser 26 is constituted, in the region of the belly fairing12, by the aircraft outer skin, which is composed of a titanium alloy,an aluminium alloy, a fibre-plastic compound material or glass-fibrereinforced aluminium. The water that is evaporated as it flows throughthe cooling channels 17 of the evaporative cooling system 16 emergesfrom the cooling channels through a steam outlet line 28, becomesdistributed in the interior of the belly fairing 12 and travels to aninner face of the aircraft's outer skin, without the necessity toprovide pipelines. The water vapour condenses on the inner face of theaircraft's outer skin, and emits the thereby released condensation heatto the environment, via the outer face of the aircraft's outer skin.

If the fuel cell 14 generates, for example, 1 MW of electrical power, itis necessary for 0.5 litres of water per second to be evaporated duringoperation of the fuel cell 14 in order to provide for proper removal ofthe heat of reaction from the fuel cell 14. The water vapour travels,via the steam outlet line 28, through the interior of the belly fairing12 to the inner face of the aircraft's outer skin, where it condenses.Thus, during operation of the fuel cell 14, approximately 0.5 litres persecond of condensed water run down on the inner face of the aircraft'souter skin.

The condensed water running down on the inner faces of the aircraft'souter skin is collected in a condensate collecting region 30. Thecondensate collecting region 30 is located in a region of the aircraft'souter-skin portion that constitutes the condenser 26, which is locatedclose to the floor, such that gravity can be utilized to collect thecondensed water. From the condensate collecting region 30, the condensedwater is conveyed, by means of the pump 18, either into thecooling-medium tank 20 or returned directly into the cooling channels 17realized in the bipolar plates of the fuel cell 14. A closedcooling-medium circuit is thereby produced.

Clearly, the components arranged in the interior of the belly fairing 12must be protected against moisture. However, since water vapour of atemperature of at least approximately 60° C. is being suppliedcontinuously to the interior of the belly fairing 12, via the steamoutlet line 28, during operation of the fuel cell 14, there is no needfor the components arranged in the interior of the belly fairing 12 tobe protected against ice and cold during flight. The belly fairing 12can be realized as a compartment accommodating the fuel cell system 10.The water vapour produced by the evaporative cooling system 16 can beused for ventilation and/or inerting of this compartment.

In order to ensure reliable operation of the fuel cell system 10 even ifproper removal, via the condenser 26, of the fuel-cell heat of reactionstored in the cooling medium water is rendered more difficult because ofhigh ambient temperatures or in the case of conditions of calm, the fuelcell system 10 further comprises an apparatus, realized in the form oftwo steam outlet valves 32, 34, for removing the cooling medium water tothe environment. The steam outlet valves 32, 34 are actuated by means ofthe electronic control unit 24. For this purpose, the control unit 24receives signals from the pressure sensor 22 and/or from a temperaturesensor 36 for measuring the temperature in the interior of the bellyfairing 12. If the pressure sensor 22 and/or the temperature sensor 36indicates/indicate that the pressure and/or the temperature in the bellyfairing 12 exceeds/exceed a predefined critical maximum value/values,the steam outlet values 32, 34 are opened by the electronic control unit24, such that water vapour delivered into the interior of the bellyfairing 12 via the steam outlet line 28, and thus also the thermalenergy stored in the water vapour, can be removed from the interior ofthe belly fairing 12 into the environment.

In order to ensure proper functioning of the evaporative cooling system16 even in the event of failure of the condenser 26 or following removalof cooling medium to the environment, provision must be made for asupply of cooling medium into the evaporative cooling system 16, i.e.into the coolant tank 20 or the cooling channels 17, that is independentof the condensing of the cooling medium evaporated as it flows throughthe cooling channels 17. For this purpose, the fuel cell system 10 has aprocess-water take-off apparatus 35, which serves to receive waterproduced by the fuel cell 14 during operation and supply it to thecoolant tank 20 of the evaporative cooling system 16.

Finally, the fuel cell system 10 has a storage system 37 for storingelectrical energy generated during operation of the fuel cell 14. Thestorage system 37 serves to intermediately store excess energy generatedby the fuel cell 14 and, if required, deliver it to loads on board theaircraft that are supplied with electrical energy by the fuel cellsystem 10. The storage system 37 can comprise, for example, asuper-capacitor or a plurality of super-capacitors.

In principle, the belly fairing 12 can also be realized as a pressurevessel. In this case, the pressure p₁ in the interior of the bellyfairing 12 can also be above the ambient pressure p_(A). Since thecooling-medium vapour is compressible, the interior of the belly fairing12 can then also serve as a storage vessel for absorbing loadfluctuations. Moreover, in the case of specific variation of thepressure p₁ in the interior of the belly fairing 12 throughcorresponding control of the pump 18, and of the heat input through thefuel cell 14, and of the steam outlet valves 32, 34, it is possible toinfluence the intensity of the heat transfer on an inner face of thebelly-fairing wall, since the heat transfer is pressure-dependent.However, design of the belly fairing 12 as a pressure vesselnecessitates a corresponding reinforcement of the belly-fairing wall,and therefore results in an unwanted increase in weight.

The fuel cell system 10 shown in FIG. 2 differs from the fuel cellsystem according to FIG. 1 in that the fuel cell 14 is realized, not asa low-temperature PEM fuel cell, but as a high-temperature PEM fuelcell. The high-temperature PEM fuel cell 14 according to FIG. 2 isnormally operated at higher operating temperatures (up to 200° C.) thanthe low-temperature PEM fuel cell 14 shown in FIG. 1.

Moreover, a valve 40, which, in its closed position, delimits a pressurezone comprising the cooling channels 17 of the evaporative coolingsystem 16, is arranged in the steam outlet line 28 that is connected tothe cooling channels 17 of the evaporative cooling system 16. Thispressure zone can be so designed that it can withstand an overpressureof several bars. By means of the pump 18, therefore, a pressure p₂ thatis higher than the pressure p₁ in the belly fairing 12 can be producedin the pressure zone comprising the cooling channels 17 of theevaporative cooling system 16. A pressure sensor, not shown in FIG. 2,can be provided to measure the pressure in this pressure zone.

In the fuel cell system shown in FIG. 2, at a pressure in the coolingchannels 17 of the evaporative cooling system 16 that corresponds to alow ambient pressure p_(A), there can occur superheating andconsequently overheating of material and damage to material in thecooling channels 17 of the evaporative cooling system 16, owing to thelarge difference between the relatively low evaporation temperature ofthe cooling medium at low pressure and the relatively high operatingtemperature of the high-temperature PEM fuel cell 14. In order toprevent this, the pressure in the cooling channels 17 of the evaporativecooling system 16 is specifically increased, by means of the pump 18, inorder so to raise the evaporation temperature of the cooling medium thatan optimum boiling behaviour of the cooling medium is effected at theoperating temperature of the high-temperature PEM fuel cell 14.

The control unit 24 can control the operating temperature of the fuelcell 14, in dependence on the pressure in the cooling channels 17 of theevaporative cooling system 16, such that the evaporation of the waterflowing by the cooling channels, by the heat of reaction generated bythe fuel cell 14 during operation, is effected in the wet-steam region.As an alternative thereto, however, the control unit 24 can also use theoperating temperature of the fuel cell 14 as a control variable, andcontrol the pressure in the cooling channels 17 of the evaporativecooling system 16, in dependence on the operating temperature of thefuel cell 14, such that the to evaporation of the water flowing throughthe cooling channels, by the heat of reaction generated by the fuel cell14 during operation, is effected in the wet-steam region. The operationof the fuel cell system 10 shown in FIG. 2 can thus be effectedirrespective of the ambient pressure p_(A) and therefore irrespective ofthe flight altitude of the aircraft. Moreover, a variation of thepressure in the cooling channels 17 of the evaporative cooling system 16renders possible control of the heat transfer in the cooling channels17. The interaction between the pump 18, the heat of reaction generatedby the fuel cell 14 during operation, the valve 40 and the steam outletvalves 32, 34 is controlled by the control unit 24 in dependence onmeteorologically determined and operational influencing variables, suchas flight altitude, load demand of an on-board electrical power system,the load state of the storage system 37, etc. In other respects, thestructure and the functioning of the fuel cell system 10 represented inFIG. 2 correspond to the structure and the functioning of thearrangement according to FIG. 1.

The fuel cell system 10 illustrated in FIG. 3 differs from thearrangement shown in FIG. 2 in that the fuel cell system 10 comprises,not only a condenser 26 for removing the fuel-cell heat of reaction tothe environment, but also a device 42, realized in the form of ade-icing installation, for utilizing the heat stored in the coolingmedium water. The de-icing installation comprises two steam lines 44,46, which branch off from the steam outlet line 28, and through whichwater vapour emerging from the cooling channels 17 of the evaporativecooling system 16 can be taken away and delivered into regions of theaircraft that are to be de-iced.

The water vapour, as it flows through the steam lines 44, 46, releasesto the regions of the aircraft to be de-iced the thermal energy storedin the vapour, and is then returned, either still in gaseous form or inliquid form, into the interior of the belly fairing 12. Pressure controlvalves 48, 50, 52, 54 are arranged in the steam lines 44, 46 for thepurpose of controlling a desired pressure in the steam lines 44, 46. Thepressure control valves 48, 50, 52, 54 are controlled in dependence onthe desired pressure p₂ in the cooling channels 17 of the evaporativecooling system 16 and a desired pressure p₃ in the steam lines 44, 46.However, the pressure p₃ in the steam lines 44, 46 cannot exceed thepressure p₂ in the cooling channels 17 of the evaporative cooling system16. A variation of the pressure in the steam lines 44, 46 enables theheat transfer in the steam lines 44, 46 to be controlled. Theinteraction between the pump 18, the heat of reaction generated by thefuel cell 14 during operation, the valve 40, the pressure control valves48, 50, 52, 54 and the steam outlet valves 32, 34 is controlled by thecontrol unit 24 in dependence on meteorologically determined andoperational influencing variables, such as flight altitude, load demandof an on-board electrical power system, the load state of the storagesystem 37, etc. and, if necessary, with specification of a priorityde-icing of the main structure.

Moreover, the fuel cell system 10 illustrated in FIG. 3 comprises twosupply installations 56 for supplying water vapour into an exhaust-gasstream of two aircraft engines 58. The water vapour can be sucked intothe exhaust-gas stream of the aircraft engines 58 through, for example,venturi tubes. The supplying of water vapour into the exhaust-gas streamof the aircraft engines 58 can reduce the pollutant emission of theengines 58. In other respects, the structure and the functioning of thefuel cell system 10 represented in FIG. 3 correspond to the structureand the functioning of the arrangement according to FIG. 2.

1. A fuel cell system comprising: a fuel cell; an evaporative coolingsystem, which is in thermal contact with the fuel cell wherein heatgenerated by the fuel cell during operation of the fuel cell is absorbedthrough evaporation of a cooling medium and is removed from the fuelcell; a device for sensing the pressure in the evaporative coolingsystem; and a control unit, which is adapted to control an operatingtemperature of the fuel cell in dependence on signals that are suppliedto the control unit from the device for sensing the pressure in theevaporative cooling system, in such a way that the cooling medium of theevaporative cooling system is transferred from a liquid to a gaseousstate of matter by the heat generated by the fuel cell during operationof the fuel cell.
 2. A fuel cell system according to claim 1 wherein thecontrol unit is set up to control the operating temperature of the fuelcell in dependence on the signals that are supplied to the control unitfrom the device for sensing the pressure in the evaporative coolingsystem, in such a way that the evaporation of the cooling medium of theevaporative cooling system by the heat generated by the fuel cell duringoperation of the fuel cell is effected in a wet-steam region of thecooling medium.
 3. A fuel cell system according to claim 1, wherein afuel-cell operating-pressure generating system is adapted to generate adesired pressure in the fuel cell, and a control unit is adapted forcontrolling the fuel-cell operating-pressure generating system.
 4. Afuel cell system according to claim 3, wherein the control unit forcontrolling the fuel-cell operating-pressure generating system isadapted to control the fuel-cell operating-pressure generating systemsuch that there is generated in the fuel cell a pressure at whichunwanted evaporation of substances and/or substance mixtures usuallypresent in liquid form in the fuel cell is prevented.
 5. A fuel cellsystem according to claim 3, wherein the control unit for controllingthe fuel-cell operating-pressure generating system is adapted to controlthe pressure in the fuel cell in dependence on the operating temperatureof the fuel cell and/or in dependence on the signals of the device forsensing the pressure in the evaporative cooling system.
 6. A fuel cellsystem according to claim 1, wherein the evaporative cooling systemcomprises a condenser, for condensing the cooling medium evaporatedduring operation of the fuel cell for the purpose of cooling the fuelcell.
 7. A fuel cell system according to claim 6, wherein the condenseris realized in the form of an outerskin cooler.
 8. A fuel cell systemaccording to claim 1, further comprising at least one device forutilizing heat stored in the cooling medium.
 9. A fuel cell systemaccording to claim 8, wherein the device for utilizing the heat storedin the cooling medium is a steam heating means, a water desalinationinstallation or a deicing installation of an aircraft.
 10. A fuel cellsystem according to claim 1, further comprising an apparatus forremoving the cooling medium to the environment.
 11. A method foroperating a fuel cell system comprising a fuel cell and an evaporativecooling system, which is in thermal contact with the fuel cell, in orderthat heat generated by the fuel cell during operation of the fuel cellis absorbed through evaporation of a cooling medium and is removed fromthe fuel cell, comprising the steps of: sensing the pressure in theevaporative cooling system by means of a pressure sensing device andcontrolling the operating temperature of the fuel cell by means of acontrol unit in dependence on signals that are supplied to a controlunit from the pressure sensing device, in such a manner that the coolingmedium of the evaporative cooling system is transferred from a liquid toa gaseous state of matter by the heat generated by the fuel cell duringoperation of the fuel cell.
 12. A method according to claim 11, whereinthe operating temperature of the fuel cell is controlled, in dependenceon the signals that are supplied to the control unit from the pressuresensing device, in such a manner that the evaporation of the coolingmedium of the evaporative cooling system by the heat generated by thefuel cell during operation of the fuel cell is effected in a wet-steamregion of the cooling medium.
 13. A method according to claim 11,wherein a desired pressure is generated in the fuel cell by means of afuel-cell operating-pressure generating system.
 14. A method accordingto claim 13, wherein the fuel-cell operating-pressure generating system,by means of a control unit for controlling the fuel-celloperating-pressure generating system, is controlled such that there isgenerated in the fuel cell a pressure at which unwanted evaporation ofsubstances and/or substance mixtures usually present in liquid form inthe fuel cell is prevented.
 15. A method according to claim 11, whereinthe control unit for controlling the fuel-cell operating-pressuregenerating system controls the pressure in the fuel cell in dependenceon the operating temperature of the fuel cell and/or in dependence onthe signals of the pressure sensing device.
 16. A method according toclaim 11, wherein the cooling medium evaporated during operation of thefuel cell for the purpose of cooling the fuel cell is condensed in acondenser.
 17. A method according to claim 16, wherein the coolingmedium is condensed by a condenser in the form of an outer-skin cooler.18. A method according to claim 11, wherein heat stored in the coolingmedium is supplied to at least one device for the utilization of theheat.
 19. A method according to claim 18, wherein the heat stored in thecooling medium is supplied to a device for the utilization of the heat,which device is realized in the form of a steam heating means, a waterdesalination installation or a de-icing installation of an aircraft. 20.A method according to claim 11, wherein the cooling medium is removed tothe environment.
 21. An aircraft comprising a fuel cell system accordingto claim
 1. 22. An aircraft according to claim 21, wherein the fuel cellsystem is arranged in a non-pressurized region of the aircraft.